Chopper chopper-stabilized instrumentation and operational amplifiers

ABSTRACT

Chopper chopper-stabilized instrumentation and operational amplifiers having ultra low offset. The instrumentation amplifiers use current-feedback, and include, in addition to a main chopper amplifier chain, a chopper stabilized loop for correcting for the offset of the input amplifiers for the input signal and for receiving the feedback of the output voltage sense signal. Additional loops, which may include offset compensation and autozeroing loops, may be added to compensate for offsets in the chopper stabilized loop for correcting for the offset of the input amplifiers. Similar compensation is disclosed for decreasing the offset in operational amplifiers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of instrumentationamplifiers.

2. Prior Art

An Instrumentation Amplifier is often made up of 3 operationalamplifiers (OpAmps). The first two amplifiers are buffer amplifiers. Thethird amplifier is an amplifier with a four-resistor bridge as afeedback network. This configuration has two main disadvantages:Firstly, the common-mode rejection ratio (CMRR) is limited by theunbalance of the resistive bridge. Secondly, the input voltagecommon-mode (CM) range cannot include the negative rail because of theoverall feedback from the output to the input by the OpAmps(“Operational Amplifiers”, Johan Huijsing, Kluwer Academic Publishers).

Therefore, the current-feedback instrumentation amplifier is a betteralternative. Its topology is shown in FIG. 1. It is excellently suitedto allow the negative or positive supply rail voltage to be includedinto the input common-mode range (“Indirect current feedbackinstrumentation amplifier with a common-mode input range that includesthe negative rail”, B. J. van den Dool et al., IEEE Journal of SolidState Circuits, Vol. 38, No. 7, July 1993, Pgs. 743-749). The reason isthat the input signal and feedback signal are independently sensed bythe voltage-to-current (V-I) converters G₃ and G₄. For instance, ifthese V-I converters are composed of identical differential P-channelpairs, the negative supply rail can be included. For obtaining a betteraccuracy and CMRR, the V-I converters can be each composed of twohigh-transconductance composite P-channel transistors with adegeneration resistor between the sources. This also improves thematching of the two identical transconductances G₃ and G₄ for betteroverall gain accuracy.

The instrumentation amplifier of FIG. 1 further consists of an outputstage G1 and an intermediate stage G2. A nested Miller compensation withC_(M11), C_(M12), C_(M21), C_(M22) provides a preferred straightroll-off of the frequency characteristic.

To obtain low offset, choppers can be inserted in the signal path aroundthe input stages, as shown in FIG. 2. With choppers, the offset canroughly be reduced by a factor 100-1000, from 10 mV to 100-10 μV. Butthere are several limitations. Firstly, a square wave at the chopperfrequency of the size of the offset referred to the input will appeararound the correct average signal value. To erase this square wave, alow-pass filter has to be placed after the instrumentation amplifier.This reduces the bandwidth of the instrumentation amplifier to below 0.1(10%) of the chopper frequency. If the chopper frequency F₁ is 10 kHz,the bandwidth will be reduced to several hundreds Hz.

Secondly, there are several effects that limit the offset reduction. Oneof them is an imperfect 50% duty cycle of the chopper frequency. Anotheris an unbalance of the charge injection in the choppers by the switchingsignal. Further, the initial offset will not fully be averaged out dueto parasitic capacitors between the first chopper inputs in combinationwith attenuation resistors at the inputs. Most of these limitations,except charge injection, would vanish if the initial offset of the inputamplifiers could be reduced by trimming or by autozeroing. Trimming isundesirable and not preferred in mass-production due to additional testtime, cost and complexity, and lack of stability over temperature andtime. One cannot simply autozero an instrumentation amplifier as wasdone in the prior art for OpAmps (U.S. Pat. No. 6,734,723, Huijsing etal.), because in accordance with FIGS. 1 and 2, the input voltage is notzero, but instead, the input stages carry the input and feedbackvoltages, respectively. In that regard, FIG. 3 presents a prior artchopper-stabilized OpAmp. Because an OpAmp is a high gain amplifier usedwith negative feedback, the closed loop differential input voltage toamplifier g₃ is zero, so that the input to chopper Ch₂ is simply theaccumulated offsets of amplifiers g₃, g₂ and g₁ as referred to the inputof amplifier g₃.

As used herein and in the disclosure and claims of the present inventionto follow, the word stability and the various other forms of the wordsometimes refer to stability in the sense of the absence of significantdrift over time and temperature, not stability in the sense of absenceof self oscillation or ringing, or hangup on either rail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art current-feedbackinstrumentation amplifier.

FIG. 2 is a block diagram of a prior art instrumentation amplifier likethat of FIG. 1, though with choppers inserted in the signal path aroundthe input stages.

FIG. 3 is a block diagram of a prior art chopper-stabilized OpAmp.

FIG. 4 is a block diagram of one embodiment of the present inventionchopper chopper-stabilized current-feedback instrumentation amplifier.

FIG. 5 is a block diagram of an embodiment similar to that of FIG. 4,but including further improvements in the chopper chopper-stabilizedcurrent-feedback instrumentation amplifier.

FIG. 6 is a block diagram of another embodiment of chopper-stabilizedcurrent-feedback instrumentation amplifier.

FIG. 7 is a block diagram of an embodiment similar to that of FIG. 6,but including further improvements in the chopper chopper-stabilizedcurrent-feedback instrumentation amplifier similar to the improvementsin the embodiment of FIG. 5.

FIG. 8 is a block diagram of an improved chopper-stabilized OpAmp.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention is shown in FIG. 4. The basicchopper current-feedback instrumentation amplifier of FIG. 2 is used asthe main instrumentation amplifier. The voltage-to-current converter G₃senses the input signal V_(in)=V_(in+)−V_(in−), while thevoltage-to-current converter G₄ takes the sense feedback output signalV_(s)=V_(s+)−V_(s−). If G₃=G4, the high loop gain of the whole amplifierforces the feedback sense voltage V_(s) to be equal and opposite to theinput voltage V_(in).

The choppers Ch₁, Ch₂ and Ch₃ chop the offset voltage of the amplifiersG₃ and G₄. The chopped offset can be regarded as a square-waveinterference voltage referred to the input voltage of amplifiers G₃ andG₄. The input voltage V_(in) is determined by an external source, andwhile generally may be a varying signal, it does not contain thesquare-wave signal. The high loop gain of the whole amplifier forces thefeedback-sense voltage V_(s) to compensate the chopped input offsetvoltage. Therefore, this square-wave chopped input offset will besuperimposed on the desired feedback sense voltage V_(s).

In the embodiment of FIG. 4, amplifiers G₇ and G₈ (voltage to currentconverters) are used to obtain a gauge to control the offset ofamplifiers G₃ and G₄. More specifically, with respect to DC levels, theclosed loop circuit settles with V_(in) and V_(s) being equal andopposite voltages. However the sense voltage V_(s) has the offset causedsquare wave on it while V_(in) does not. Consequently the output currentof amplifier G₇ plus the output current of amplifier G₈ will simply bethe square wave caused by the offset of amplifier G₃. Thus the resultingcurrent represents the square-wave chopped input offset voltagecomponent from V_(s), and largely suppresses the desired input andfeedback sense voltages.

Next the combined output currents of amplifiers G₇ and G₈ are rectifiedinto a DC current by the chopper Ch₄. This DC current represents theinput offset voltage. Next this DC current is integrated by anintegrator amplifier G₆, with the integrator output voltage beingconverted into a current by G₅ and added to the output currents ofamplifiers G₃ and G₄ in order to gradually cancel the input offsetvoltage of these amplifiers. Since the offsets are at most very slowlyvarying, such as by temperature or time variations, in general theresponse of this offset control loop need not be particularly fast, andgenerally is intentionally given a time constant much longer than thechopper frequency period so as to be a substantially fixed offsetcompensation during each chopper period. Note that the integrator hasthe effect of integrating the rectified square wave on the sense voltageV_(s), no matter how small, so that, neglecting other sources of error,the offset control loop settles when the offset is eliminated, and isnot limited to the gain within the control loop.

The chopper chopper-stabilized current-feedback instrumentationamplifier of FIG. 4 can still be improved on 3 issues. These furtherimprovements are depicted in FIG. 5.

The sense amplifiers G₇ and G₈ each have an offset voltage. This offsetis represented as an offset current at their output and further choppedby chopper Ch₄ (FIG. 4) into a square wave current. This current isintegrated into a triangle shaped voltage by the integrator G₆ and addedto the output by amplifier G₅. By chopper Ch₃, the triangle waveform isreshaped into a sawtooth referred to the feedback sense V_(s). This isan undesired signal. Also the offset of amplifiers G₇ and G₈ togetherwith an imperfection of the 50% duty cycle of chopper Ch₄ will result ina DC component, which cannot be distinguished from the offset ofamplifiers G₃ and G₄. Therefore, the offset of G₇ and G₈ should bereduced.

Thus the first main improvement is to reduce the offset of amplifiers G₇and G₈. Therefore, an autozero phase through multiplexer MUX₁ during onefull clock cycle is introduced. In this phase the multiplexer allows theoutput of amplifiers G₇ and G₈ to be integrated by amplifier G₉. Theamplifier G₁₀ feeds the integrated offset back and corrects for it.

The offset of amplifier G₉ should be low because it builds charge acrossthe parasitic capacitances at the output of amplifiers G₇,₈, which willlater be discharged by a different offset of the integrator G₆. Thisresults in an incorrect sensing of the offset of amplifiers G₃ and G₄,similar to the offset of amplifier G₆, as described hereinafter, and asquare wave residue. To reduce the offset of amplifier G₉, a chopperstabilisation loop is built around it consisting of the choppers Ch₅ andCh₆, the sense amplifier G₁₁, the integrator G₁₂ and correctionamplifier G₁₃.

If integrator G₆ has an input offset voltage, this voltage will show asa square wave before the chopper Ch₄. This will charge and discharge theparasitic capacitors at the output of amplifiers G₇ and G₈. These chargepulses will be integrated into a DC voltage by integrator G₆. This DCvoltage cannot be distinguished from the DC integrator voltage thatrepresents the offset of amplifiers G₃ and G₄. As a result, the offsetof amplifiers G₃ and G₄ is not compensated correctly, and a square waveby the choppers Ch₂ and Ch₃ will remain. Therefore, the offset ofintegrator G₆ has to be reduced.

Thus the second main improvement is to reduce the offset of amplifierG₆. For that purpose, a secondary offset detection and correctioncircuit has been added similar to the circuitry G₈, Ch₄, G₆, G₅. Thesecondary offset sense and correction loop consists of a sense amplifierG₁₄, a chopper Ch₇, an integrator G₁₅ and a correction amplifier G₁₆.The sense amplifier G₁₄ senses the square wave before chopper Ch₄ causedby the offset of amplifier G₆. Chopper Ch₇ redirects the square wave andthe integrator G₁₅ integrates the offset caused by amplifier G₆. Thecorrection amplifier G₁₆ closes the loop.

However, this secondary loop also needs a third order correction.Firstly, the offset of amplifier G₁₄, being chopped by Ch₇, creates atriangle wave at the output of the integrator G₁₅. This triangle isadded through amplifiers G₁₆ and G₅ and referred to the feedback inputthrough amplifiers G_(3,4) and chopper Ch₃ as a sawtooth waveform. Thisis undesirable. Therefore, an autozero loop has been placed aroundamplifier G₁₄ through multiplexer MUX₂, integrator G₂₃ and correctionamplifier G₂₄. This is similar to MUX1, integrator G₉ and correctionamplifier G₁₀, to correct the offset of amplifiers G₇ and G₈.

The offset of integrator G₁₅ introduces a square wave before chopperCh₇. The parasitic output capacitance at the output of amplifier G₁₄creates charge pulses, which are rectified by chopper Ch₇ and integratedagain by integrator G₁₅ into an incorrect correction signal, which lookslike an offset of the original integrator G₆, resulting in a square waveresidue. Therefore, another or third order correction loop is created tocorrect the offset of amplifier G₁₅. This loop consists of the senseamplifier G₂₀, chopper Ch₈, integrator G₂₁, and correction amplifierG₂₂.

Finally, the offset of amplifier G₂ in the main amplifier will show asan input offset, but reduced by the voltage gain of amplifiers G₃ andG₄. If the offset of amplifier G₂ is 10 mV, and the voltage gain ofamplifiers G₃ and G₄ is 1000, there still is an offset of 10 μV. Henceit is good to also reduce the offset of amplifier G₂.

Moreover, the offset of amplifier G₂ results in charge peaks introducedby the parasitic capacitances at the output of amplifiers G₃, G₄ and G₅in combination with the chopping activity of chopper Ch₁. Also for thispurpose, it is desirable to reduce the offset of amplifier G₂.

The offset of amplifier G₂ results in a residual offset and spikes.Therefore, a sense and correction loop is built around amplifier G₂,consisting of a sense amplifier G₁₇, chopper Ch₉, integrator G₁₈, andcorrection amplifier G₁₉. This is similar as the loop formed byamplifiers G_(8,9), chopper Ch₄, integrator G₆ and correction amplifierG₅.

It appears possible to simplify the methods hereinbefore described foruse in chopper-stabilized amplifiers. A basic architecture for achopper-stabilized current-feedback instrumentation amplifier is shownin FIG. 6. Because there are no choppers in the main feed forward signalpath, no square-wave offset related signal can be found at the inputvoltage V_(s) of amplifier G₄, though the offsets are still present.

However, using choppers Ch₂ and Ch₃ to chop the input voltage V_(in) andfeedback sense voltage V_(s), and converting the chopped voltages V_(in)and V_(s) into currents by amplifiers G₇ and G₈ G₈ and subtracting theoutput currents of amplifiers G₇ and G₈ (V_(in) and V_(s) are equal andopposite differential voltages), a current signal representing thechopped offset of amplifiers G₃ and G₄ is obtained. Chopping this againby chopper Ch₄, a DC signal representing the offset of G₃ and G₄ isobtained. Integrating this signal by integrator G₆ and adding it by acorrection amplifier G₅ to the output summing node of amplifiers G₃ andG₄ compensates for the offset.

There is one drawback in regard to the chopper chopper-stabilizedversion of FIG. 6 however. Specifically, if the gains of amplifiers G₇and G₈ are not equal, DC input signals at V_(in) and V_(s) cannot bedistinguished from the offset. Thus the offset correction is DC signaldependent.

This can also be interpreted as a gain error ΔA=G_(7/8)-G_(3/4) at verylow frequencies, where the gain of the correction path through G₇ and G₈and G₆ and G₅ dominates the gain of the straight path through G₃ and G₄.But these drawbacks may be overcome by auto-trimming or bydynamic-element matching techniques.

In the same way as the basic chopper chopper-stabilized instrumentationamplifier of FIG. 4 was further improved by second-order and third-ordercorrection loops, the chopper-stabilized current-feedbackinstrumentation amplifier of FIG. 6 can be further improved. This isshown in FIG. 7. Most of the correction loops have been described withrespect to FIG. 5. The multiplexer MUX1 together with amplifiers G₉ andG₁₀ autozero amplifiers G₇ and G₈, while chopper Ch₅, amplifier G₁₁,chopper Ch₆, integrator G₁₂, and amplifier G₁₃ chopper stabilizeintegrator G₉. Similarly, amplifier G₁₄, chopper Ch₇, integrator G₁₅ andamplifier G₁₆ chopper-stabilize integrator G₆, while multiplexer MUX2,integrator G₂₃ and amplifier G₂₄ autozero amplifier G₁₄, and alsoamplifier G₂₀, chopper Ch₈, integrator G₂₁ and amplifier G₂₂chopper-stabilize amplifier G₁₅. The main purpose of the loop aroundamplifier G₂ in FIG. 5 was to reduce the offset of amplifier G₂ so thatspikes caused by chopper Ch₁ were reduced.

Now that chopper Ch₁ of FIGS. 3 and 4 has been removed in FIG. 7, thechopper-stabilized loop around G₂ might not be necessary anymore. But ifin any case this loop is still desired, for instance to reduce theeffect of offset of amplifier G₂ on the input, chopper Ch₁ now needs tobe placed inside the correction loop together with amplifier G₁₇,chopper Ch₉, integrator G₁₈ and amplifier G₁₉, as shown in FIG. 7.

The instrumentation amplifier of FIG. 7 can be reduced to an OpAmp byeliminating amplifier G₄, chopper Ch₃ and amplifier G₈, resulting in theimproved chopper-stabilized OpAmp of FIG. 8. In that regard, theoperation of the circuit is identical to that explained with respect toFIG. 7 with the exception that because it is used as an OpAmp, asexplained before, in use, the negative feedback will force thedifferential input to V_(in) to zero, so that the only DC component inthe input V_(in) will be the accumulated offsets of amplifiers g₃, g₂and g₁ as referred to the input of amplifier g₃. Consequentlycancellation of the DC component of the input signal required ininstrumentation amplifiers and accomplished by amplifier G₄, chopper Ch₃and amplifier G₈ in FIG. 7 is not required in the OpAmp of FIG. 8.

Thus there has been disclosed herein ultra low offset, low spikeartifact instrumentation amplifiers that have a main chopper amplifierchain (backwards numbered) amplifiers G₁ and G₂, chopper Ch₁, amplifiersG_(3,4) and chopper Ch_(2,3), with a first order offset cancellationloop with amplifier G₅, integrator G₆, chopper Ch₄ and amplifiersG_(7,8). Also disclosed as possible improvements are up to threesecond-order cancellation loops comprising; multiplexer MUX1, integratorG₉ and amplifier G₁₀; amplifier G₁₄, chopper Ch₇, integrator G₁₅ andamplifier G₁₆; and amplifier G₁₇, chopper Ch₉, integrator G₁₈ andamplifier G₁₉. Further disclosed as possible improvements are up tothree third order cancellation loops; chopper Ch₅, amplifier G₁₁,chopper h₆, integrator G₁₂ and amplifier G₁₃; multiplexer MUX₂,integrator G₂₃ and amplifier G₂₄; and amplifier G₂₀, chopper Ch₈,integrator G₂₁ and amplifier G₂₂.

Further disclosed is the application of the inventive aspects of thepresent invention chopper-stabilized current-feedback instrumentationamplifiers to chopper-stabilized OpAmps. The exemplary embodiments aredescribed with respect to differential amplifiers, though may berealized as single ended amplifiers also, that is, as single input,single output amplifiers. Also in the embodiments disclosed, two outputstages are shown, though in some cases, such as in the case ofamplifiers that are lightly loaded, a single stage may be used,dispensing with the use of amplifier G₂ and Miller compensationcapacitors CM₂₁ and CM₂₂. Also amplifier G5 may be an attenuator, eitheran amplifier with a gain of less than one, or simply resistors forconverting the integrator output to a current for input to the currentsumming point or for attenuation. Additional Miller compensated, nestedamplifiers may also be incorporated as desired. Thus while certainpreferred embodiments of the present invention have been disclosed anddescribed herein for purposes of illustration and not for purposes oflimitation, it will be understood by those skilled in the art thatvarious changes in form and detail may be made therein without departingfrom the spirit and scope of the invention.

1. A method of amplification comprising: a) amplifying an amplifierinput signal and a sense voltage feedback signal and adding theamplified signals; b) amplifying the added signal to provide anamplifier output; c) attenuating the amplifier output to provide a sensevoltage feedback signal; d) chopping and amplifying the input signal andthe sense voltage feedback signal, and adding the result; e) choppingthe result in d) and integrating the chopped result by a firstintegrator; and, f) adding a signal responsive to the result of theintegration in e) with the signals added in a).
 2. The method of claim 1wherein the amplifying in b) comprises two cascaded stages ofamplification, each stage having Miller compensation from the amplifieroutput in b) to an input of the respective stage.
 3. The method of claim1 wherein the signals as added in a) and the signals added in f) arecurrents added by coupling respective currents to a current summingpoint.
 4. The method of claim 1 wherein the result of integration in e)is amplified or attenuated before the adding in f).
 5. The method ofclaim 1 wherein in d), amplifiers used for the amplifying are autozeroedby a second order loop.
 6. The method of claim 5 wherein the autozeroingat least comprises an amplifier coupled as a second integrator, andfurther comprising chopper stabilizing the amplifier coupled as thesecond integrator.
 7. The method of claim 6 wherein the autozeroingfurther comprises a correction amplifier having an input coupled to anoutput of the second integrator and having an output added with theresult in d).
 8. The method of claims 1 or 5 further comprised ofdetecting and correcting the offset of the amplifier connected as thefirst integrator with a secondary offset correction loop.
 9. The methodof claim 8 wherein the secondary offset correction loop includes a senseamplifier having an input coupled to the input of the chopper used ine), and further comprising autozeroing the sense amplifier.
 10. Themethod of claim 9 wherein the secondary offset correction loop includesan amplifier coupled as a third integrator, and detecting and correctingthe offset of the amplifier connected as the third integrator with athird order offset correction loop.
 11. The method of claim 1 whereinthe amplification in b) includes sensing and correcting the offset of atleast a first stage of the amplification.
 12. The method of claim 5wherein the amplification in first stage of the amplification.
 13. Themethod of claim 8 wherein the amplification in b) includes sensing andcorrecting the offset of at least a first stage of the amplification.14. The method of claim 1 wherein the amplifying in b) comprises twostages of amplification, each stage having Miller compensation from theamplifier output in b) to an input of the respective stage.
 15. A methodof amplification comprising: a) chopping an input signal; b) chopping asense voltage feedback signal; c) amplifying and adding the choppedsignals of a) and b); d) chopping the added signal of c) and amplifyingthe chopped signal to provide an amplifier output; e) attenuating theamplifier output to provide a sense voltage feedback signal; f)amplifying the input signal and the sense voltage feedback signal andadding the result; g) chopping the result in f) and integrating thechopped result by an amplifier coupled as a first integrator; and, h)adding a signal responsive to the result of the integration to thesignals added in c).
 16. The method of claim 15 wherein the amplifyingin d) comprises two stages of amplification, each stage having Millercompensation from the amplifier output in d) to an input of therespective stage.
 17. The method of claim 15 wherein the signals asamplified in c) and the signals added in h) are currents added bycoupling to a summing point.
 18. The method of claim 15 wherein theresult of integration in g) is amplified or attenuated before adding inh).
 19. The method of claim 15 wherein in f), amplifiers used for theamplifying are autozeroed.
 20. The method of claim 19 wherein theautozeroing includes an amplifier coupled as a second integrator. 21.The method of claim 20 further comprising chopper stabilizing theamplifier coupled as the second integrator.
 22. The method of claims 15or 19 further comprised of detecting and correcting the offset of theamplifier connected as the first integrator with a secondary offsetcorrection loop.
 23. The method of claim 22 wherein the amplification ind) includes sensing and correcting the offset of at least a first stageof the amplification.
 24. The method of claim 22 wherein the secondaryoffset correction loop includes a sense amplifier having an inputcoupled to the input of the first integrator through the chopper of g),and further comprising autozeroing the sense amplifier.
 25. The methodof claim 24 wherein the secondary offset correction loop includes anamplifier coupled as a third integrator.
 26. The method of claim 25further comprising correcting the offset of the amplifier connected asthe third integrator with a third order offset correction loop.
 27. Themethod of claim 25 wherein the amplification in d) includes sensing andcorrecting the offset of at least a first stage of the amplification.28. A method of amplification comprising: a) amplifying an amplifierinput signal and adding the result with an offset compensation signal toprovide an added signal; b) amplifying the added signal of a) to providean amplifier output; c) chopping and amplifying the amplifier inputsignal; d) chopping the result in c) and integrating the chopped result,the integration being done with an amplifier coupled as a firstintegrator, and detecting and correcting the offset of the amplifier ofthe first integrator with a secondary offset correction loop; e)providing as the offset compensation signal of a), a signal responsiveto the result of the integration in d); f) autozeroing the amplifierused in c), the autozeroing including an amplifier coupled as a secondintegrator.
 29. The method of claim 28 further comprising chopperstabilizing the amplifier of the second integrator.
 30. The method ofclaim 28 wherein the secondary offset correction loop includes a senseamplifier having an input coupled to the input of a chopper used in d),and autozeroing the sense amplifier.
 31. The method of claim 28 whereinthe secondary offset compensation loop has an amplifier coupled as athird integrator, and correcting the offset of the amplifier of thethird integrator with a third order offset compensation loop.
 32. Themethod of claim 29 wherein the secondary offset correction loop includesa sense amplifier having an input coupled to the input of a chopper usedin d), and autozeroing the sense amplifier.
 33. The method of claim 29wherein the secondary offset compensation loop has an amplifier coupledas a third integrator, and correcting the offset of the amplifier of thethird integrator with a third order offset compensation loop.
 34. Themethod of claim 28, 29, 30, 31, 32 or 33 wherein the amplifying in b)comprises two stages of amplification, each stage having Millercompensation from the amplifier output in b) to an input of therespective stage.
 35. The method of claim 34 wherein the amplificationin b) comprises sensing and correcting the offset of at least a firststage of the amplification.
 36. The method of claim 28 wherein thesignals as added in a) are currents added by coupling respectivecurrents to a current summing point.
 37. The method of claim 28 whereinthe result of integration in d) is amplified before the adding in a).